Final answer:
The PR interval reflects the delay in passage of the electrical impulse from the SA node to the AV node on an ECG.
Step-by-step explanation:
The PR interval reflects the duration from the beginning of atrial depolarization (the P wave) to the initiation of the QRS complex on an ECG. It is clinically relevant as it measures the delay in passage of the electrical impulse from the sinoatrial (SA) node to the atrioventricular (AV) node. If there is a delay in this passage, it would be visible in the PR interval.